1. Field of the Invention
The present invention relates to a process for producing an electroluminescent element. More particularly, the present invention relates to a process for producing an electroluminescent element, which can realize the provision of an electroluminescent element with high luminescence efficiency even by photolithography.
2. Background Art
Electroluminescent (hereinafter referred to as “EL”) elements have drawn attention as self-luminous planar display elements. In EL elements, holes and electrons injected from opposed electrodes are combined with each other within a luminescent layer, energy in this combination excites a fluorescent material within the luminescent layer, and luminescence of color depending upon the fluorescent material takes place.
Among others, organic thin-film EL displays using organic materials as luminescent materials are very advantageous, for example, in that high-brightness luminescence can be realized even at an applied voltage of slightly lower than 10 V, that is, high luminescence efficiency can be realized, and luminescence can take place in a simple element structure. Accordingly, it is expected that these organic thin-film EL displays could be applied to advertisements where luminescent display of a specific pattern is performed, as well as to other low-price simplified displays.
Such organic EL elements generally have a basic structure comprising a first electrode layer and a second electrode layer stacked on top of each other through an organic EL layer, and, in general, the first electrode layer and the organic EL layer are patterned to form an element which can provide different luminescent colors.
Various patterning methods for these EL elements have been proposed, and examples thereof include a method in which a luminescent material is vapor deposited through a shadow mask, a method in which selective coating is carried out by an ink jet recording method, a method in which a specific luminescent coloring matter is decomposed by ultraviolet irradiation, and a method using screen printing. These methods, however, could not have realized an EL element having high luminescence efficiency and high light take-out efficiency and could not have realized high-definition patterning in a simple production process.
To overcome the above problem, a process for producing an EL element has been proposed in which the luminescent layer is patterned by photolithography (see, for example, Japanese Patent Laid-Open No. 170673/2002). According to this process, unlike patterning of the luminescent layer by the conventional vapor deposition, for example, a vacuum system having a highly accurate alignment function is not necessary, and, thus, the EL element can be produced relatively easily at low cost. Further, in this process, unlike the case where an ink jet recording method is used, for example, the formation of an auxiliary structure in patterning and the pretreatment of the substrate are not necessary. Accordingly, this process is advantageous in that the production process is simple and, at the same time, the accuracy of the patterning by the photolithography is higher than the delivery accuracy of an ink jet recording head.
The above production process of an EL element using the photolithographic method generally comprises the following steps. Specifically, a material for luminescent layer formation is coated onto a substrate to form a luminescent layer, and a photoresist layer is covered on the luminescent layer. The photoresist layer is exposed through a photomask so that a desirably patterned luminescent layer can be formed. Development is then carried out with a photoresist developing solution, followed by washing to remove the photoresist layer in its exposed areas. The luminescent layer in its parts exposed on the surface of the assembly as a result of the removal of the photoresist layer are removed by etching or the like to form a desirably patterned luminescent layer. The above steps are repeated to pattern luminescent layers corresponding to respective colors. Finally, the remaining photoresist layer provided on the luminescent layers is separated by a peeling liquid to remove the photoresist layer, whereby patterning of luminescent layers corresponding to respective colors, for example, red R, green G, and blue B is carried out. An EL element is then prepared through the step of forming a second electrode on the luminescent layers of the respective colors.
The EL element in which the luminescent layers have been patterned by photolithography, however, sometimes has unsatisfactory luminescence characteristics, and, thus, the development of an EL element having higher luminescence efficiency has been desired.